1. Field
The present invention relates generally to wireless communications, and more specifically to varying the number of taps of an adaptive equalizer based on Doppler frequency.
2. Background
Today there are a variety of sophisticated wireless communications systems in use around the world. One such communication system is a code division multiple access (CDMA) system which conforms to the “TIA/EIA/IS-95 Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System,” hereinafter referred to as the IS-95 standard. The CDMA system allows for voice and data communications between users over a terrestrial link. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled “SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS,” and U.S. Pat. No. 5,103,459, entitled “SYSTEM AND METHOD FOR GENERATING WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM,” both assigned to the assignee of the present invention and incorporated by reference herein. The “TIA/EIA/IS-2000 Standard” describes a next generation cdma2000 multi-carrier 1× and 3× air interface specification, hereinafter referred to as the cdma2000 standard.
In the CDMA system, communications between users are conducted through one or more base stations. In this specification, a base station refers to the hardware with which user terminals communicate. A first user terminal communicates with a second user terminal by transmitting data on a reverse link to a base station. The base station receives the data and can route the data to another base station. The data is transmitted on a forward link of the same base station, or a second base station, to the second mobile station. The forward link refers to transmission from the base station to a user terminal and the reverse link refers to transmission from the user terminal to a base station. In IS-95 systems, the forward link and the reverse link are allocated separate frequencies.
Given the growing demand for wireless data applications, the need for very efficient wireless data communication systems has become increasingly significant. The IS-95 standard is capable of transmitting traffic data and voice data over the forward and reverse links. A method for transmitting traffic data in code channel frames of fixed size is described in detail in U.S. Pat. No. 5,504,773, entitled “METHOD AND APPARATUS FOR THE FORMATTING OF DATA FOR TRANSMISSION,” assigned to the assignee of the present invention and incorporated by reference herein. Further, a high data rate (HDR) system that provides for high rate packet data transmission in a CDMA system is described in detail in the “TIA/EIA/IS-856—cdma2000 High Rate Packet Data Air Interface Specification” (hereinafter referred to as the HDR standard), as well as in co-pending U.S. patent application Ser. No. 08/963,386, entitled “METHOD AND APPARATUS FOR HIGH RATE PACKET DATA TRANSMISSION”, filed Nov. 3, 1997, now U.S. Pat. No. 6,574,211, issued Jun. 3, 2003, and assigned to the assignee of the present invention and incorporated by reference herein.
Various techniques are employed in systems such as HDR to mitigate the effects of inter-symbol interference (ISI) arising from multipath propagation and imperfect filtering. For example, multiple receiver antennas can be used to exploit diversity against fading. This allows for signal processing in the spatial domain. Adaptive equalizers can also be employed at the receiver to cancel noise and interference in multipath environments. Spatial domain filtering and adaptive equalizers are well known techniques, and their use at the receiver can offer improved performance in systems such as HDR.
Adaptive equalizer performance can be characterized in a number of ways. Two important performance criteria are adaptation speed and minimizing mean square error (MMSE). Adaptation speed is particularly important in those environments where channel interference varies rapidly over time. The tracking ability of the equalizer will depend, at least in part, on the rate at which the equalizer coefficients (or taps) converge as the channel profile changes. Adaptation speed can be affected by a variety of factors. One factor is the selection of an algorithm for adapting the equalizer coefficients. These algorithms are often based on the criteria of minimizing the MSE between known pilot symbols and the equalizer's estimates of these pilot symbols. Two common examples of adaptive MMSE algorithms are the least-mean-square (LMS) algorithm and the recursive-least-squares (RLS) algorithm. For the same number of taps, an equalizer updated using the RLS algorithm will converge faster than an equalizer updated using the LMS algorithm, but the RLS algorithm is more complex to implement.
Another factor affecting adaptation speed is the number of taps in the equalizer, referred to herein as the equalizer length. Adaptation speed is inversely proportional to equalizer length simply because shorter equalizers have fewer coefficients that require updating.
The MMSE associated with an adaptive equalizer determines, among other things, the equalizer's effectiveness at reducing ISI once the filter coefficients converge. MMSE also depends on a number of factors including equalizer length. Longer equalizers will, in general, have a lower steady-state MMSE and will therefore be more effective at reducing ISI. But they will require longer periods of time to converge. Shorter equalizers will converge more quickly but they will have a higher MMSE and will therefore be less effective at reducing ISI.
The channel dynamics should therefore be considered when designing an adaptive equalizer, and particularly when selecting the equalizer length. In general, a shorter equalizer will perform better than a longer equalizer when the multipath channel is varying quickly. This is because shorter equalizers are better able to track the time-varying multipath. Conversely, longer equalizers are more desirable when the multipath channel is varying slowly because the longer equalizer has sufficient time to converge and does a better job at reducing interference.
However, it becomes difficult to set an appropriate equalizer length for those environments where the channel varies quickly over certain periods of time but slowly over other periods. Multipath interference on a mobile telephony channel may often be modeled as such. Shorter equalizers will be able to effectively track the quickly varying channel, but will not perform as well during slowly varying times. Longer equalizers will do a good job of reducing interference during slowly varying periods, but may not be able to effectively track quickly varying channels. An average length equalizer could also be selected that performs sub-optimally under all conditions.
There is therefore a need in the art for an adaptive equalizer capable of effectively reducing ISI in those environments where the rate of change of the channel varies over time.